diff.c: flush stdout before printing rename warnings

The diff output is buffered in a FILE object and could still be partially buffered when we print these warnings (directly to fd 2). The output is messed up like this worktree.c | 138 +- worktree.h warning: inexact rename detection was skipped due to too many files. | 12 +- wrapper.c | 83 +- It gets worse if the warning is printed after color codes for the graph part are already printed. You'll get a warning in green or red. Flush stdout first, so we can get something like this instead: xdiff/xutils.c | 42 +- xdiff/xutils.h | 4 +- 1033 files changed, 150824 insertions(+), 69395 deletions(-) warning: inexact rename detection was skipped due to too many files. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>